Electrical apparatus

ABSTRACT

A micro-switch comprises at least one coil ( 22 ) of elongate electrically conductive material mounted on the support means ( 16 ). A movable actuator portion ( 20 ) of electrically conductive material is attached to the support means ( 16 ) so as to be movable against resilient means ( 10 ) of the micro-switch and so as to be adjacent to the coil ( 22 ). Pulse inductive circuitry ( 30, 32, 34, 36, 43   a   , 52, 54 ) is connected to the said at least one coil ( 22 ), and constructed to switch from one condition to another when the actuator portion ( 20 ) is moved against the force of the resilient means ( 10 ) beyond a predetermined threshold point as indicated by pulsoinductive monitoring effected by the pulse inductive circuitry ( 30, 32, 34, 36,43   a   , 52, 54 ). As a result of this construction, there are no physical contacts which are brought into and out of contact with one another to effect the change in the electrical condition of the micro-switch.

ELECTRICAL APPARATUS

A first aspect of the present invention relates to a micro-switch havinga movable actuator portion secured to support means of the micro-switchvia resilient means, the micro-switch being so constructed as to beswitched from one electrical condition to another when the actuatorportion is moved against the force of the resilient means beyond apredetermined threshold point.

Hitherto, such constructions of micro-switch have contacts which arebrought into electrical contact with one another or which are taken outof electrical contact with one another depending upon the position ofthe actuator portion. This defines the changing condition of themicro-switch. Usually, the position of the actuator portion to cause abreak between the contacts is different from that which causes thecontacts to be electrically connected to one another. There is thus ahysteresis in the operation in such a previously proposed micro-switch.

A disadvantage of the foregoing construction of micro-switch is that,especially because of wear and tear in the contact parts, themicro-switch is unreliable and may change over a period of time asregards the positions of the actuator part which causes make and/orbreak of the contacts.

The present invention seeks to provide a remedy.

According to a first aspect of the present invention, a micro-switchcomprises at least one coil of elongate electrically conductive materialmounted on support means, a movable actuator portion of electricallyconductive material attached to the support means so as to be movableagainst resilient means of the micro-switch and so as to be adjacent tothe coil, and pulse inductive circuitry connected to the said at leastone coil, and constructed to switch from one condition to another whenthe actuator portion is moved against the force of the resilient meansbeyond a predetermined threshold point as indicated by pulse inductivemonitoring effected by the pulse inductive circuitry.

As a result of this construction, there are no physical contacts whichare brought into and out of contact with one another to effect thechange in the electrical condition of the micro-switch.

It will be appreciated in this context that pulse induction involves ameasure of the voltage of other electrical parameter across the coil ata time after the effects of an energizing pulse on the coil would havesubstantially completely died away in the absence of the actuatorportion.

Preferably, the pulse inductive circuitry is constructed to switch fromthe said another condition to the said one condition when the actuatorportion is moved with the force of the resilient means beyond apredetermined threshold point as indicated by pulse inductive monitoringeffected by the pulse inductive circuitry. This predetermined thresholdpoint may be the same position as the predetermined threshold point atwhich the circuitry is changed from the said one condition to the saidanother condition when the actuator portion is moved against the forceof the resilient means.

Preferably, the pulse inductive circuitry is so constructed to provide ameasurement of the voltage or other electrical parameter across the coilat respective first and second instants of time after an energizingpulse.

Preferably, the pulse inductive circuitry is further constructed tocheck whether the actuator portion is moved beyond the threshold pointreferred to with the actuator portion being moved against the force ofthe resilient means, at the said first instant, and to check whether theactuator portion is moved beyond the threshold point referred to withthe actuator portion moving with the force of the resilient means, atthe said second instant. In this way, the hysteresis behavior of theaforementioned previously proposed micro-switch can be mimicked.

It is desirable for measurement cycles, each comprising an energizingpulse followed by a measurement, to be repeated continuously. Themeasurement may then comprise an average of successive measurements ofrespective successive measurement cycles.

The period between successive measurement cycles may be for a first,relatively long, time interval unless and until a movement of theactuated portion is indicated by at least one of the measurement cycles,whereupon the period between successive measurement cycles may bereduced.

As a check against the possibility that a stray signal has given a falsemeasurement, a plurality of successive further values of a measurementfor successive cycles may be checked by the circuitry to ascertainwhether the first indication of movement was false or not. If it was,then the period between successive cycles may be immediately returned tothe relatively long period. If it was not, the shorter period betweenmeasurement cycles may be maintained by the circuitry.

In a relatively simple construction of the micro-switch, the saidresilient means comprises a relatively springy arm secured at one end ofits ends to the support means and providing the actuating portion at itsother end.

Advantageously, the actuator portion lies on the axis of the said atleast one coil. A relatively strong signal is obtainable if the actuatorportion is able to enter the coil interior.

Advantageously, the arm is substantially at right angles to the axis ofthe coil.

The micro-switch may be made in a relatively simple and inexpensivefashion if the whole of the arm including the actuator portion is madeof the same electrically conductive material and the actuator portion isprovided beyond a bend in the arm. Preferably, this bend effects a turnof the material of the arm of about 90□.

In a further development of this aspect of the invention, there is atleast one further predetermined threshold point beyond which theactuator portion may be moved against the force of the resilient meansto effect a switching of the pulse inductive circuitry to a furthercondition.

Provision may be made to remove the actuator portion to enable ameasurement to be made after an energizing pulse has issued, resultingfrom the environment of the micro-switch as opposed to the position ofthe actuator portion, thereby to correct the threshold setting for theenvironment in which the micro-switch is placed.

Desirably, the period of each energizing pulse is substantially equal tothe actuator time constant.

The circuitry may be so constructed as to take a further measurement ata third instant, to check that the coil and the circuitry and theassociated components are present and working at a time when theactuator portion is fully withdrawn.

It is desirable for the circuitry to be further constructed to checkthat the voltage or other electrical parameter which is measured acrossthe coil during a measurement cycle is substantially zero at a time whenit would be expected that the signal has reached zero after anenergizing pulse.

Diagnostic energizing pulses may be issued in addition to themeasurement energizing pulses to confirm that the circuitry is presentand correct.

A second aspect of the present invention relates to a position sensorcomprising at least two coils of elongate electrically conductivematerial which are spaced apart from one another, or which diverge fromone another in the sense that their respective axes are at an angle toone another and respective portions of the coils are substantiallycontiguous, the position sensor further comprising an actuator portionarranged to be moved within a region which is within or adjacent to thesaid at least two coils, and pulse inductive circuitry connected to saidat least two coils to provide a signal which is dependent upon theposition of the actuator portion relative to the coils.

One such device is discussed in WO-00/25093. A disadvantage of such apreviously proposed position sensor is that it is relatively susceptibleto changes in temperature and its circuit tolerances and tolerances asregards the positioning of the position sensor components are relativelylow.

The present invention seeks to provide a remedy.

Accordingly, a second aspect of the present invention is directed to aposition sensor comprising at least two coils of elongate electricallyconductive material which are spaced apart from one another, or whichdiverge from one another in the sense that their respective axes are atan angle to one another and respective portions of the coils aresubstantially contiguous, the position sensor further comprising anactuator portion of electrically conductive material arranged to bemoved within a region which is within or adjacent to the said at leasttwo coils, and pulse inductive circuitry connected to the said at leasttwo coils to provide respective signals having respective values, eachindicative of the position of the actuator portion relative to therespective one of the said at least two coils, in which the pulseinductive circuitry is constructed to provide a signal having a valueobtained substantially by dividing the difference between the saidrespective values by the sum of the said respective values.

An advantage of such a construction is that the value of the signalobtained is dimensionless.

It is desirable for measurement cycles, each comprising an energizingpulse followed by a measurement, to be repeated continuously. Themeasurement may then comprise an average of successive measurements ofrespective successive measurement cycles.

The period between successive measurement cycles may be for a first,relatively long, time interval unless and until a movement of theactuator portion is indicated by at least one of the measurement cycles,whereupon the period between successive measurement cycles may bereduced.

As a check against the possibility that a stray signal has given a falsemeasurement, a plurality of successive further values of measurementsfor successive cycles may be checked by the circuitry to ascertainwhether the first indication of movement was false or not. If it was,then the interval between successive cycles may be immediately returnedto the relatively long period. If it was not, the shorter intervalbetween measurement cycles may be maintained by the circuitry.

In one form of the position sensor, the two coils have their axes spacedapart and parallel to one another, and the actuator portion is generallyU-shaped having respective ends adjacent respectively to the coils.Preferably, in this construction the ends lie on the respective axes ofthe coils.

Alternatively, the two coils may be spaced apart but have axes in commonwith one another, the actuator portion being movable along the axisbetween the coils.

Such a construction is particularly effective if the actuator portioncomprises a sleeve of non-magnetically permeable electrically conductivematerial wound around a rod of magnetically permeable material.

In one valuable construction of position sensor, there are amultiplicity of coils, with a multiplicity of associated pulse inductivecircuits, which are constructed to detect which of the two coils theactuator portion is for the time being closest to, and to effect theprovision of a signal on the basis of the values of the signals fromthose respective coils.

For both aspects of the present invention, it is desirable for thecreation of the energizing pulse to be effected by the switching of afield effect transistor of the circuitry.

The present invention extends to control apparatus incorporating amicro-switch or a position sensor embodying the present invention.

Examples of micro-switches and position sensors made in accordance withthe present invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic perspective part cut-away representation of amicro-switch embodying the present invention;

FIG. 2 is a simplified diagrammatic perspective view of parts of themicro-switch shown in FIG. 1;

FIG. 3 is an explanatory graph;

FIG. 4 is a diagrammatic perspective view of a modified micro-switchembodying the present invention;

FIG. 5 is a further explanatory graph;

FIG. 6 shows a block circuit diagram of the electrical circuitry ofeither of the micro-switches shown in FIGS. 1 and 4;

FIG. 7 shows a further explanatory graph;

FIG. 8 shows a modified form of the circuitry of either one of themicro-switches shown in FIGS. 1 and 4;

FIGS. 9 a and 9 b show side and end views respectively of an alternativeconstruction of a part of either one of the micro-switches shown inFIGS. 1 and 4;

FIGS. 10 a to 10 e show, respectively, side, bottom, end, top andperspective views of part of a position sensor embodying the presentinvention;

FIGS. 11 and 12 show further explanatory graphs;

FIGS. 13 a to 13 f show possible modifications to the part shown in FIG.10 a to 10 e, Figures of the same letter being of corresponding view;

FIG. 14 shows a perspective view of a further modified position sensorembodying the present invention;

FIG. 14 a shows a perspective view of a modified part of the positionsensor shown in FIG. 14;

FIGS. 15 and 18 show respective further embodiments of the presentinvention in diagrammatic form;

FIG. 16 shows a perspective view of parts of a further position sensormade in accordance with the present invention;

FIGS. 17 and 19 to 21 show respective further perspective views ofrespective further position sensors made in accordance with the presentinvention; and

FIGS. 22 to 25 show respective further circuits of respective differentposition sensors embodying the present invention.

The micro-switch shown in FIG. 1 comprises a steel arm 10 which isgenerally fixed by a screw 12 at one of its ends 14 to a base 16. Thearm 10 is bent at its end 18 further from the screw 12, to provide anactuator portion 20 at that end 18 generally at right angles to the restof the arm 10. A coil 22 is also mounted on the base 16 with its axisgenerally at right angles to that of the arm 10, the actuator portion 20lying on the axis of the coil 22. The actuator portion 20 can be movedfrom its position illustrated in FIG. 1, against the resilience of thearm 10, along the axis of the coil 22, further into the interiorthereof. A circuit such as is disclosed below may be incorporated intoan element, such as base 16, illustrated in figure 1. further into theinterior thereof.

A simplified view of parts of the micro-switch shown in FIG. 1 is shownin FIG. 2, comprising the actuator portion 20, and a single generallysquare interlaced coil 22 as parts of the micro-switch. The response ofthe apparatus is plotted on the vertical axis against linear axialposition of the actuator portion 20 along the axis on the coil 22, inFIG. 3.

FIGS. 4 and 5 correspond respectively to FIGS. 2 and 3, but with atapered actuator portion 20. The graph shows a higher degree oflinearity for greater displacement.

The block circuit diagram shown in FIG. 6 shows circuitry used inconjunction with the coil 22. This comprises a system clock 30 connectedto deliver clock pulses to a pulse generator 32. This delivers an 80μsec switching pulse to a switch 34 so that, during that time, theswitch is closed and the voltage of about 5 volts is connected to oneend of the coil 22, the other being earthed. Also connected across thecoil are voltage measuring means 36 comprising a differential amplifier38, a switch 40 and buffer amplifier 42 connected in series with oneanother with an output signal 44 being taken from the output of thebuffer amplifier 42, the positive input to the differential amplifierbeing connected to the non-earthed end of the coil 22 and the negativeinput of the differential amplifier being connected to a point betweentwo series connected resistors 46 and 48 constituting a feedback fromthe buffer amplifier 42 and connected to earth. The positive connectionto the differential amplifier 38 is also connected to earth by aresistor 50.

A time delay 52 is also connected to the pulse generator 32, and a pulsegenerator 54 generating a pulse of approximately 3 μsec is connected toreceive a signal from a delay 52 and cause the switch 40 to be closedfor that pulse period.

The system clock 30 causes the pulse generator 32 to close the switch 34for a period of approximately 80 μsec.

This energizes the coil 22 for that period such that the voltage acrossthe coil has a step function as shown in the graph in FIG. 7. When thispulse ends at time t0 in FIG. 7, the self-inductance of the coil 22causes the voltage across it to fall sharply to a negative value of amagnitude well in excess of the 5 volts it had initially, whereafter attime t1 it starts to rise again and to reach zero value at about time t2following an exponential curve C1 between time t1 and t2. However, withthe presence of the electrically-conductive portion 20, it follows thebroken curve C2, in which the decay of a negative voltage across thecoil 22 is still exponential (shown very diagrammatically in FIG. 7),but is slowed down so that the voltage does not come to zero value againuntil about time t3, well after time t2.

The actual measure of this decay influence is measured by that part ofthe circuitry shown in the box 36 of FIG. 6. Thus, the switch 40receives the pulse which closes it for about 3 μsec, about 10 μsec afterthe coil 22 was de-energized (by which time the excitation energy hascompletely died away). This therefore provides a measure of the voltageacross the coil 22 at time t4, about 20 μsec after time t0 and lastingfor about a period of 3 μsec. Thus, the measure of the decay influenceat time t4 occurs at a time when the voltage across the coil 22 wouldhave been substantially zero had the actuator portion 20 been absent. Inthe Figure, this voltage is very nearly zero, which is sufficient.Provided more time is available, it would be preferable to position t4where that voltage is zero (although in reality the voltage never isexactly zero).

The signal from the buffer 42 is received by a first input of acomparator 43, the other input of which is connected to receive a valuefrom a comparator reference 43 a. The latter provides a predeterminedthreshold value. If the signal at the first input of the comparator 43exceeds that threshold value, then the output signal at the output 44changes from a low value to a high value, or vice versa.

FIG. 8 shows simplified circuitry for the micro-switch of FIG. 1. Theenergizing pulse is delivered to the coil 22 via a switching fieldeffect transistor (FET) 80 having an input terminal 82 connected to a 4volt voltage supply and an output terminal 84 connected to one end ofthe coil 22 via a resistor 86. The switching terminal 88 of the FET 80is connected to pulse generator with a control circuit 90 via a resistor92. A further resistor 94 and a capacitor 96 are connected in seriesacross the coil 22. The other end of the coil 22 is connected to ground,and a diode 98 may be connected in parallel across the capacitor 96 forconduction towards the FET 80. The control circuit 90 is connected toobserve the voltage across the capacitor 96.

In the modification of the micro-switch shown in FIGS. 9 a and 9 b, thecoil 22 is provided by a hollow cylinder 370, with spacers 372 and asingle elongate coil 374 wound around the cylinder 370 with suitableslots (not shown) being formed in the spacers 372 to enable the windingto be continuous along the length of the cylinder 370. In this case, themovable electrically-conductive portion 20 (not shown in FIGS. 9 a and 9b) would extend into the interior of the cylinder 370, without touchingit, and would move in its longitudinal direction.

FIGS. 10 a to 10 e show parts of a position sensor 214 and theirrelative position in relation to an electrically-conductive actuatorportion 200. The sensor 214 shown in these Figures comprises a hollowbox 216 of nylon or other electrically non-conductive plastics material,molded into the shape of an open bottomed box. The box is generallyelongate. A first transverse slot 222 is machined across the outside ofthe top of the box. In each side of the box, on the outside thereof, aremachined two slanting slots 224 which extend downwardly from one end ofthe slot 222 to respective corners of the box, with the angle betweenthe two slots 224 being approximately 100°. Lastly, there are two endslots 226 machined across the bottoms of the end walls of the box 220.

Two coils 228 of copper filament or other electrically-conductive wireare wound around the box, each winding being generally rectangular withone side of the rectangle seated in the slot 222, the opposite side ofone of the coils being in one of the slots 226 and the opposite side ofthe other coil being in the other slot 226 with the other sides of thetwo coils seated in the slanting slots 224. Thus, the two coils 228diverge from one another, from their sides which are contiguous andwhich are both seated in the slot 222, with an angle of about 100°between them.

As can be seen from FIG. 10 e, the electrically-conductive portion 200has an upper end received within the interior of the box 220 withouttouching any part of that box, this end being within a volume defined bythe coils 228. The coils surround that volume, and the volume extendsbetween the coils.

FIG. 11 shows output plotted against actuator position when the latteris composite, providing two actuator portions which are physically fixedin position relative to one another and which are provided withrespective different coil portions the outputs from which aresubtracted. The different curves show different relative positions ofthe two actuator portions, one of which can be seen to provide asubstantially linear output for the full movement range. This is alsoshown in FIG. 12, where the composite actuator is secured to anaccelerator foot pedal, and the output in volts is shown as a functionof rotation of the pedal in degrees.

The modification to the position sensor shown in FIGS. 13 a to 13 e,comprises an increase in the width of the box 220, and the provision oftwo pairs of coils, each pair being wound in substantially the samefashion as in the two coils of the position sensor part shown in FIGS.10 a to 10 e, and each pair being orthogonally arranged to the otherpair. The reference numerals used in FIGS. 13 a to 13 e correspond tothose used in FIGS. 10 a to 10 e. It will be appreciated that with sucha construction, the position of the electrically-conductive portion 200can be determined with respect to two degrees of freedom, so that it ispossible to determine the position of the electrically-conductiveportion 200 both along the length of the box 220 and also across itswidth. One such application for such a position sensor is to determineboth the relative position along two orthogonal axes of a joystick, theoutputs from the position sensor being used to position a tool and/or amachine tool table in both of two orthogonal axes, or to vary the speedof movement of the tool and/or machine tool table in these directions.In another such application, such a joystick provided with such aposition sensor could be used to control a radio-controlled vehicle ortoy.

Numerous variations and modifications to the illustrated embodiments mayoccur to the reader without taking the result outside the scope of thepresent invention. For example, the box 20 with the coils 28 may beenclosed in an aluminium or copper casing to minimize the effect ofexternal fields whilst still enabling useful measurements to be made.

The modification shown in FIG. 13 f comprises the coils 228 spaced apartfrom one another but sharing a common axis, the actuator portion 200comprising a non-magnetically permeable electrically conductive sleeve1200 surrounding a magnetically permeable rod 1220.

The modified apparatus shown in FIG. 14 has coiled portions 14 a, a mainone of which is elongate transversely of its winding axis and two endcoil portions overlapping the ends of the main elongate coiled portion,the latter being movable into and out of a tubular actuator portion 20.The latter may be modified so that it has an inverted U-shape as shownin FIG. 14 a.

In the modified apparatus of FIG. 15, the coil 14 a is also elongate andthe actuator portion 20 is tubular, being a hollow piston rod of apiston and cylinder arrangement, so that the apparatus of which the coiland actuator portions are parts determines the position of the pistonrod of this arrangement.

FIG. 16 shows a possible construction for the coil 14 a as two coilportions spaced apart, having a common winding axis, and beingelectrically connected in series with one another. These coils allow fora short overall construction.

FIG. 17 shows a construction having two coils 14 a which are spacedapart, having a common winding axis, but being connected separately to aposition sensor (not shown in FIG. 17), so as to provide signals whichare subtracted from one another to give a substantially linear response,that being further enhanced by the actuator portion 20, which iscomposite and which has two tapered ends each movable into and out ofthe volumes surrounded respectively by the coil portions.

In the apparatus of which a part is shown in FIG. 118, the coils 14 aare arranged as shown in FIG. 17, but the actuator portion comprises asteel ball 812, which is free to roll on a part spherical dish 890, sothat the apparatus is able to measure tilt, and could comprise a tiltswitch. This arrangement may be enclosed and within oil for lubricationand damping.

In the arrangement of FIG. 19, the coils 14 a are placed alongside oneanother with the respective axes of winding parallel with one another,and the actuator portion 20 is again composite, comprising a yoke with atapered end on the axis of one of the coils 14 a and another tapered endon the axis of the other of the coils 14 a, the yoke being arranged tobe movable linearly along a direction parallel to the coil axes, theends of the actuator portion 20 extending in opposite directions so thatas one end approaches its coil 14 a, the other leaves its coil 14 awhilst travelling in the same direction, and vice versa. The same effectis obtainable with a motion of the yoke about an axis which is displacedfrom the coils and which is parallel to a line passing through thecenters of the coils 14 a.

In the modification shown in FIG. 20, the yoke is generallysemi-circular, with its ends generally at the respective centers of thecoils 14 a, possible movement of the yoke being a rocking motion aboutthe centre of the circle on which it lies.

In the construction shown in FIG. 21, the actuator portion is hollow,comprising two generally trapezoidal sides 721 connected above by abridging portion 722. This is linearly movable to receive, to anincreasing or decreasing extent, two coils arranged as in FIGS. 19 and20, the sides 721 being parallel to the coils 14 a.

Each of the coils 14 a in the arrangements shown in FIGS. 17 to 21 maycomprise the composite coil construction shown in FIG. 16.

The portions of a composite actuator portion could be separate.

The actuator portions may be made of steel, aluminium, brass or otherelectrically-conductive metal alloy or other electrically-conductivematerial.

The electrically-conductive material of the actuator portion isadvantageously magnetically permeable, as is steel for example.

The circuitry to which the coils 14 a or 228 of the position sensorsshown in FIGS. 10 a to 10 e, 13 a to 13 e, 13 f, and 14 to 21 is showndiagrammatically in FIG. 22. It comprises two circuits 960 and 962, eachbeing the same as the circuit shown in FIG. 6 or FIG. 8, these twocircuits being connected to the two coils, or two of the coils, of theposition sensor respectively. The outputs of the circuits 960 and 962are connected to respective inputs of an operator circuit 963. If theoutputs from the circuits 960 and 962 have the values A and Brespectively, the operator circuit is such as to provide at its output964 a signal having the value (A−B)/(A+B). The latter output may besmoothed by a capacitor 986.

When the apparatus is in use, the circuitry shown in FIG. 22 operatesfor each coil with the pulses being transmitted to the two coilsasynchronously so that when one is energized, the other is not, and viceversa, and such that there is a delay period between each pulse whenneither winding is energized to avoid a measurement by one of thewindings interfering with that of the other.

FIG. 23 shows the circuitry of FIG. 22 in greater detail, withcorresponding parts of the circuitry in the FIGS. 6 and 23 bearing thesame reference numerals, save that where a part of the circuitry in FIG.23 relates to one of the coils, it has the suffix a, and where a part ofthe circuitry in FIG. 23 relates to the other coil, it has the suffix b.

The circuitry in FIG. 24 comprises the circuitry of FIG. 8, adapted fortwo coils. The function of the operator circuit of FIG. 22 is performedby the circuitry within the control circuit 90. Use of the referencenumerals of FIG. 24 corresponds to that for FIG. 8, save that thesuffices a and b are used for those parts of-the circuitry of FIG. 24relating to the two coils respectively. A switch 97 is provided toenable observation of the voltages across both capacitors 96 a and 96 bby the control circuit 90.

The circuitry shown in FIG. 25 corresponds to that of FIG. 24, but for amultiplicity of coils. The control circuit here is provided with means(not shown) for determining which of the two coils are providing thehighest output, and for obtaining the value (A−B)/(A+B) from the twocoils.

Means (not shown) may be provided for all the illustrated embodiments toascertain a reading given in the absence of the actuator portion 20 or200, and thereafter to modify the resulting measurement. For example, ifthere are background measurements for the two or for two of the coils ofone of the position sensors a and b respectively, the output value maybe adjusted by calculating the value (A−a−B+b)/(A−a+B−b).

1. A micro-switch comprising a movable actuator portion of electricallyconductive material attached to support means so as to be movableagainst resilient means of the micro-switch, wherein the micro-switchfurther comprises at least one coil of elongate electrically conductivematerial mounted on the support means, such that the movable actuatorportion is movable against the resilient means so as to be adjacent tothe coil, and pulse inductive circuitry connected to the said at leastone coil, and constructed to switch from one condition to another whenthe actuator portion is moved against the force of the resilient meansbeyond a predetermined threshold point as indicated by pulse inductivemonitoring effected by the pulse inductive circuitry, wherein saidinductive circuitry comprises a pulse generator that delivers aswitching pulse to a pulse switch that is connected to apply a voltageto said at least one coil to provide an energizing pulse to said atleast one coil such that when the energizing pulse ends. theself-inductance of said at least one coil causes the voltage across itto fall to a negative value of a magnitude well in excess of the voltageit had initially, and wherein the inductive circuitry further comprisesmeasuring means connected across the said at least one coil to measurethe inductance voltage there across at a time when the excitation energyhas died away, being the inductance voltage owing to the presence of themovable actuator portion.
 2. A micro-switch according to claim 1,wherein the pulse inductive circuitry is constructed to switch from thesaid another condition to the said one condition when the actuatorportion is moved with the force of the resilient means beyond apredetermined threshold point as indicated by pulse inductive monitoringeffected by the pulse inductive circuitry.
 3. A micro-switch accordingto claim 2, wherein the said predetermined threshold point is the sameposition as the predetermined threshold point at which the circuitry ischanged from the said one condition to the said another condition whenthe actuator portion is moved against the force of the resilient means.4. A micro-switch according to claim 1, wherein the pulse inductivecircuitry is so constructed as to provide a measurement of the voltageacross the coil at respective first and second instants of time after anenergizing pulse.
 5. A micro-switch according to claim 4, wherein thepulse inductive circuitry is further constructed to check whether theactuator portion is moved beyond the said threshold point with theactuator portion being moved against the force of the resilient means,at the said firs: Instant, and to check whether the actuator portion ismoved beyond the threshold point with the actuator portion moving withthe force of the resilient means, at the said second instant.
 6. Amicro-switch according to claim 1, wherein the pulse inductive circuitryis constructed to carry out measurement cycles, each comprising anenergizing pulse followed by a measurement, repeated continuously.
 7. Amicro-switch according to claim 6, wherein the pulse inductive circuitryeffects a measurement of the voltage across the coil comprising anaverage of successive measurements of respective successive measurementcycles.
 8. A micro-switch according to claim 7, wherein the periodbetween successive measurement cycles is a first, relatively long, timeinterval unless and until a movement of the actuated portion isindicated by at least one of the measurement cycles, whereupon theperiod between successive measurement cycles is reduced.
 9. Amicro-switch according to claim 7, wherein the pulse inductive circuitryis such that a plurality of successive further values of a measurementfor successive cycles is checked by the circuitry to ascertain whetherthe first indication of movement was false or not, and so tat if it was,the period between successive cycles is immediately returned to therelatively long period, and if it was not, the shorter period betweenmeasurement cycles is maintained by the circuitry.
 10. A micro-switchaccording to claim 1, wherein the actuator portion lies on the axis ofthe said at least one coil.
 11. A micro-switch according to claim 1,wherein the actuator portion is movable to enter the coil interior. 12.A micro-switch according to claim 1, wherein the said resilient meanscomprises a relatively springy arm secured at one of its ends to thesupport means and providing the actuator portion at its other end.
 13. Amicro-switch according to claim 12, wherein the arm is substantially atright angles to the axis of the coil.
 14. A micro-switch according toclaim 12, wherein the arm including the actuator portion comprises anelectrically conductive material and the actuator portion is providedbeyond a bend in the arm.
 15. A micro-switch according to claim 14,wherein the bend effects a turn of the material of the arm of about 90degrees.
 16. A micro-switch according to claim 1, wherein it is soconstructed that there is at least one further predetermined thresholdpoint beyond which the actuator portion may be moved against the forceof the resilient means to effect a switching of the pulse inductivecircuitry to a further condition.
 17. A micro-switch according to claim1, wherein the actuator portion is removable to enable a measurement tobe made after an energizing pulse has issued, resulting from theenvironment of the micro-switch as opposed to the position of theactuator portion, thereby to correct the threshold setting for theenvironment in which the micro-switch is placed.
 18. A micro-switchaccording to claim 1, wherein the period of each energizing pulse issubstantially equal to the actuator portion dine constant.
 19. Amicro-switch according to claim 4, wherein the circuitry is soconstructed is to take a further measurement at a third instant, tocheck that the coil and the circuitry and the associated components arepresent and working at a time when the actuator portion is fullywithdrawn.
 20. A micro-switch according to claim 1, wherein the pulseinductive circuitry is further constructed to check that the voltagewhich is measured across the coil during a measurement cycle issubstantially zero at a time when it would be expected that the signalhas reached zero after an energizing pulse.
 21. A micro-switch accordingto claim 1, wherein the pulse inductive circuitry is constructed toissue diagnostic energizing pulses in addition to the measurementenergizing pulses to confirm that the circuitry is present and correct.22. A micro-switch according to claim 1, wherein the pulse inductivecircuitry creates an energizing pulse to be effected by the switching ofa field effect transistor of the circuitry.
 23. Control apparatusincorporating a micro-switch as claimed in claim 1.